School sex education textbooks may have to be dramatically changed if gay marriage gets the go-ahead in the upcoming referendum.

But a key flashpoint could be school texts that have to be newly illustrated -- with pictures of gay couples with their children.

There is a fear some parents, who are strongly opposed to same-sex marriage, may object to the use of such pictures and illustrations.

"We're coming into a referendum and everything needs to change. There will always be someone who doesn't like particular images,'' says Lynda O'Shea of the National Parents' Council. "They have every right to dislike whatever they see.

"But at the end of the day as long as the image is not crude or rude, I don't think most people will have an issue with it.

"The world is changing quickly, and we either keep up with it or stay in the stone age.''

Sandra Irwin-Gowran, director of education policy change at the Gay and Lesbian Equality Network (GLEN), said the education system should reflect the changing nature of marriage in Ireland.

"If you're talking about families in schools -- you shouldn't just talk only about a married heterosexual family,'' she said.

"You should cover the diversity of family types that exist, whether it's single parents, adopted parents or same-sex parents."

She said "normal stuff'' should be reflected in school books, and that would include illustrations reflecting homosexual, as well as heterosexual parents, with their children.

A spokesperson for Folens, one of the country's leading publishers of school books, said "it always strive to create content that is educational, engaging, and is reflective of changes in Irish society at the time of going to print''.

A Department of Education spokesperson said school books are produced by commercial publishers and "in general" it has no oversight of their contents. However, it said where a school covers issues such as civil marriage, it is to be presumed this is done by way of providing relevant "factual information".

While polls suggest the referendum on gay marriage is likely to be passed, Justice Minister Alan Shatter has said the controversial issue of adoption by same-sex couples and civil partners will be legislated for regardless of the outcome.

A draft Children and Family Relationships Bill will be discussed by Cabinet shortly and will then be debated by the relevant Oireachtas committee within a matter of weeks.